When the tip of a piston operated pipette is placed in a fluid, prior to intake, a volume of air is trapped within the tip and the piston chamber. This volume of air is normally referred to a the "dead volume" (DV) and is generally a constant which is determined by the pipette tip and of air passages and chambers in the pipette nozzle. The DV may be much larger than the fluid volume being inputted and/or dispensed.
When fluid is drawn into the pipette tip, a fluid column of height h is created. The weight of this fluid column is supported by a pressure difference between the dead volume and atmospheric pressure. Since this pressure within the dead volume is less than atmospheric pressure, and since, in accordance with Boyle's Law, the product of pressure and volume is constant (i.e., PV=K), this reduction in pressure of the dead volume causes a small increase in the dead volume.
This change in volume is normally relatively small, and has heretofore generally been ignored when utilizing a pipette. However, since the DV is larger than the fluid volume, and in some systems much larger than the fluid volume, even a small percentage change in DV can cause appreciable errors in fluid volume. Therefore, in high precision applications, particularly applications for an electronic pipette such as that shown in U.S. Pat. No. 4,821,586 Apr. 18, 1989, entitled "Programmable Pipette", the error caused by changes in the dead volume results in unacceptable errors in the quantity of fluid or fluids drawn into or dispensed from the pipette. While in most applications, this error will be less than 2%, as will be discussed in greater detail later, the percentage of this error grows larger for small volumes, and may approach 7-10% for very small volumes. It is therefore desirable that this error be taken into account when performing high precision pipetting operations. This is true whether the pipette is being operated in normal mode where metering is done on intake, or in reverse mode where metering is done on dispense.
While the error resulting from changes in hydrostatic pressure on the dead volume may be a problem when a single fluid is being pipetted, it can be an even greater problem when a number of fluids are being sequentially drawn into a single tip and then dispensed to be mixed. Since the error is nonlinear, it can result in the ratio of the fluids being mixed being incorrect and, thus, either in the improper dilution of a given sample or in a resulting mixture being other than that desired.
Other error sources also exist in pipette operation which may need to be compensated for to achieve high precision operation. For example, when a metered sample is expelled, some small quantity of fluid may remain in the pipette even after an overblow operation. This phenomenon, which results from fluid adhesion to the walls of the pipette tip, surface tension and the like, may be approximated as a constant in some applications.
It is therefore desirable that a method and apparatus be provided for permitting the error resulting from the nonlinear change in dead volume in a pipette to be taken into account when either pipetting a single fluid or when pipetting a number of fluids in sequence, and that this method and apparatus be usable to compensate for other error phenomenon.